Field of the Invention
The present invention relates to using optical peak power reduction of laser pulses for semiconductor inspection and metrology systems, and in particular to using a beam splitter and one or more mirrors to generate an optimized pulse multiplier.
Related Art
The illumination needs for inspection and metrology are generally best met by continuous wave (CW) light sources. A CW light source has a constant power level, which allows for images or data to be acquired continuously. However, at many wavelengths of interest, particularly ultraviolet (UV) wavelengths, CW light sources of sufficient radiance (power per unit area per unit solid angle) are not available, are expensive or are unreliable.
A pulsed light source has an instantaneous peak power level much higher than the time-averaged power level of a CW light source. However, if a pulsed laser is the only available, or cost-effective, light source with sufficient time-averaged radiance at the wavelength of interest, then using a laser with a high repetition rate and wide pulse width is best. The higher the pulse repetition rate, the lower the instantaneous peak power per pulse for the same time-averaged power level. The lower peak power of the laser pulses results in less damage to the optics and to the sample or wafer being measured, as most damage mechanisms are non-linear and depend more strongly on peak power rather than on average power.
In some applications, an additional advantage of an increased repetition rate is that more pulses are collected per data acquisition or per pixel leading to better averaging of the pulse-to-pulse variations and better signal-to-noise ratios. Furthermore, for a rapidly moving sample, a higher pulse rate may lead to a better sampling of the sample position as a function of time, as the distance moved between each pulse is smaller.
The repetition rate of a laser subsystem can be increased by improving the laser medium, the pump system, and/or its driving electronics. Unfortunately, modifying a UV laser that is already operating at a predetermined repetition rate can require a significant investment of time and money to improve one or more of its constituent elements, and may only improve the repetition rate by a small increment. Furthermore increasing the repetition rate of the fundamental laser in a UV laser reduces the peak power of the fundamental. This reduces the efficiency of the frequency conversion (which is necessarily a non-linear process) and so makes it harder to generate high average UV power levels.
Therefore, a need arises for a practical, inexpensive technique to improve the repetition rate of a UV laser that operates on the output of the laser.